Flattop Mountain SNOTEL Snowpack: Water Year 2003

These materials prepared by the Climate Change in Mountain Ecosystems Program -- U.S. Geological Survey, Glacier Field Station.  Dan Fagre, Program Coordinator, email: dan_fagre@usgs.gov.  For more information, visit the Glacier Field Station - Climate Change in Mountain Ecosystems program (CCME) web site. 

The Flattop Mountain SNOTEL (SNOw TELemetry) station is one of nearly 600 similar stations operated throughout the western United States by the Natural Resources Conservations Service (NRCS, U.S. Department of Agriculture). These stations measure and record Snow Water Equivalent (SWE), which is the weight of snow water equivalent to inches of water. Most SNOTEL sites also measure temperature and precipitation; many now measure snow depth as well. Various agencies and organizations use the data from the stations to forecast water availability, floods, and avalanche hazard. 

The Flattop Mountain SNOTEL station has operated since October 1st, 1969. To help calculate water storage and availability during agricultural growing seasons, the data are compiled by water year; a water year runs from 1 October - 30 September. SWE records from the station extend for 33 water years (1970-2002). Precipitation records from the station start water year 1979, with temperature records starting water year 1983. 

The Flattop Mountain SNOTEL station is located at approximately 6300 feet in elevation on Flattop Mountain, a high plateau between the Lewis and Livingston Ranges in Glacier National Park, Montana. The site is three air miles south of the Continental Divide. During the winter, complex combinations of weather and terrain determine snowfall at the site. Westerly weather systems predominate, bringing moisture from the Gulf of Alaska or Pacific Ocean. Less frequent northerly systems spill drier Arctic air through passes on the Continental Divide as they slide south along the Rocky Mountain Front. Flattop Mountain's position between the Livingston and Lewis Ranges amplifies the effects of these large-scale weather systems; the two ranges rise 2-4000 feet above the site and orographically wring moisture from both westerly and upslope easterly storms. Flattop Mountain is a useful indicator of snowfall throughout Glacier National Park because it is subject to the factors that influence conditions elsewhere in the park.

The graph below shows the current water year SWE at Flattop Mountain compared to SWE from other significant water years. The data plotted includes: 

• 33-year long-term average SWE, from 1970 - 2002 

• The water year of maximum SWE - 1972 

• The water year of minimum SWE - 2001 

• The 2003 water year SWE

• The 2003 water year snow depth 

• The 2002 water year SWE

A more detailed PDF file of this graph is available for printing or viewing at SWE Comparison Graph.

A PDF file of a graph showing SWE, snowdepth, and temperature for the 2003 Water Year at Flattop is available at 2003 SWE, depth and temperature.

Since the last update of this page (June 2nd), the snowpack at Flattop Mountain SNOTEL has continued melting. Conditions were dry the first part of the 13-day period, with moderate, seasonable temperatures throughout the period. Temperatures remained above freezing except on June 3rd and 4th, when minimum temperatures dipped slightly below freezing to .6 C (30.9 F) and .5 C (31.1 F). Average temperatures ranged from 5.4 C (41.7 F) on June 6th to 8.3 C (46.9 F) on June 15th. Though the weather was cooler than the end of May, conditions were warm enough for sustained melting, with the station recording at least .5" SWE loss each day. The station lost 1" or more of SWE on four days - June 4th, 7th, 14th, and 15th; the greatest daily loss was 1.2" on June 15th. The SWE loss over the 13-day period totaled 10.6"; the station recorded no SWE accumulation. The snowmelt over the period is slightly faster than is typical for this time of year. The station recorded rain on 6 days in the second half of the period, for a precipitation total of 1.5". The snow depth at the station dropped 24.4" over the period, with the greatest drop (3.1") on June 7th. 

The SWE at Flattop Mountain SNOTEL appears to have peaked twice this water year. The first peak was 44.1" on April 17-20th, with a second peak at the same total May 10th. 44.1" represents 94% of the 33-year (1970-2002) average peak of 46.85". The SWE on April 17-20th and May 10th was 95-97% of the 33-year average for those dates. The first peak occurred 7-10 days earlier than the 33-year average of April 27th; the second occurred 13 days after that average date, and 2 days after last season's peak.

While SWE accumulation at Flattop was below average both at the peak and on most days throughout the season, the recent melt keeps the snowpack below average for this time of year. As of midnight, June 16th, SWE at Flattop stands at 21.4" and the snow depth is 41.6". The SWE represents 87% of the 33-year average for this date (24.6"). For comparison, Flattop recorded 47" SWE (191% of average and 2.9" higher than the 2003 peak SWE) and 106.2" snow depth on this date last season (Water Year 2002). The maximum SWE on this day (Julian Day 152) over the past 33 seasons is 49.2" in Water Year 1972. The minimum SWE was 0" in 1987; Flattop recorded 0" on June 13th that year, the earliest meltout in the last 34 water years. 

The June 15th SWE at Flattop Mountain SNOTEL is not representative of the SWE at most other SNOTEL stations in the Flathead River Basin. Most stations are recording less than 1" of SWE. Most stations above 5900 feet elevation are showing 75-92% of NRCS 30-year averages, with
data from 3 stations missing. The two sites closest to the Continental Divide are showing SWE totals well below average (9 and 19%). The SWE data now averages 71% throughout the basin, with precipitation at 83% of average, though those figures do not include data from 3 stations. 

The end of an El Nino pattern may explain the sudden jump in SWE accumulation that occurred at Flattop Mountain in March this season. Flattop recorded a gain of 11.4" of SWE during March, which represents 26% of the season's peak SWE. Typically, SWE does not accumulate so rapidly at the site during March; on average Flattop records 6" of SWE gain in March, or 13% of the average peak SWE. In its May 19th El Nino/ENSO Discussion, the National Weather Service notes that warm episode conditions rapidly dissipated in the tropical Pacific during March and April 2003. The June 12th discussion reports that oceanic and atmospheric conditions in the tropical Pacific during May indicate that a transition from El Nino to La Nina conditions is progressing. Equatorial Sea Surface Temperatures (SSTs) were near or below normal east of the date line, equatorial easterlies near or stronger than normal, and subsurface temperatures below average in the eastern Pacific. The NWS concluded from these conditions and model forecasts that La Nina (cold episode) conditions are likely to develop over the next few months. For more information, see the climate prediction center's El Nino/ENSO Discussion.